The present invention relates to a liquid crystal display device and, more particularly, to an improvement in a field effect liquid crystal display device for time-multiplexed driving.
A conventional so-called twisted nematic liquid crystal display device has a 90.degree. twisted helical structure of a nematic liquid crystal having positive dielectric anisotropy and sealed between two substrates having transparent electrodes arranged thereon in desired display patterns. Polarizing plates are arranged on outer surfaces of the substrates such that polarizing axes thereof become perpendicular or parallel to the major axes of the liquid crystal molecules adjacent to the substrates.
In order to twist the liquid crystal molecules between the two substrates through 90.degree., orienting layers are formed said electrodes and exposed surfaces of the substrates by coating polyimide resin and making numerous fine grooves by rubbing the coated surfaces which contact the liquid crystal molecules by a cloth along one direction. In this case, the major axes of the liquid crystal molecules adjacent to the surface become parallel to this one direction (i.e., a rubbing direction). Two rubbed surfaces are spaced apart so as to oppose each other while their rubbing directions are crossed by 90.degree.. These rubbed substrates are then sealed with a sealing agent, and a nematic liquid crystal having positive dielectric anisotropy is filled in a space formed between the substrates. Therefore, the major axes of the liquid crystal molecules are twisted through 90.degree. between the substrates. The resultant liquid crystal cell is sandwiched between a pair of polarizing plates with their polarizing axes substantially parallel or perpendicular to the major axes of liquid crystal molecules adjacent thereto, respectively. In a conventional reflective type liquid crystal display device which is most frequently used, a reflector is disposed on the outer surface of the lower polarizing plate. Light incident on the upper surface of the device is linearly polarized by the polarizing plate or polarizer. In a portion of a liquid crystal layer which is not applied with a voltage, the plane of polarization of the linearly polarized light is rotated through 90.degree. along the helical structure and is transmitted through the lower polarizing plate. The light is then reflected by the reflector and returns to the upper surface of the device. However, in a portion of the liquid crystal layer which is applied with a voltage, where the helical structure is destroyed, the plane of polarization of the linearly polarized light will not be rotated. Therefore, the linearly polarized light transmitted through the upper polarizing plate is blocked by the lower polarizing plate and will not reach the reflector. In this manner, electrical signals can be converted into optical images in accordance with the presence or absence of an electrical potential applied across the liquid crystal layer.
The twisted nematic type liquid crystal display device (hereinafter referred to as "TN-LCD" for short), owing to its merits such as low driving voltage, low power consumption, small thickness, and light weight, has found extensive utility in wrist watches, desk computers, various industrial measuring instruments, and automotive instruments.
The dot matrix type TN-LCD which is capable of displaying letters and figures has long been arousing much interest as useful for terminal components in portable computers and various data processing units. At present, the grades of 64.times.480 picture elements, 128.times.480 picture elements, etc. adapted to operate at the duty factor of 1/64 are already on the market. The demand in market, however, is shifting to LCD's of still greater contents of display and information density such as those of 200.times.640 picture elements and 256.times.640 picture elements which are equivalent in display capacity to the cathode-ray picture tubes. For such LCD's to be commercially feasible, they are required to be effectively driven in a highly time-multiplexed fashion of the order of duty factor of 1/100 or 1/128.
Time-multiplexed driving will be briefly described with reference to a dot matrix display. As shown in FIG. 1, Y stripe electrodes (signal electrodes) 33 and X stripe electrodes (scanning electrodes) 34 are formed on the lower and upper substrates (not shown), respectively. Pixels (picture elements), liquid crystal portions at intersections of the X and Y electrodes 34 and 33 are chosen to be in an ON state or an OFF state so as to display characters or the like. N scanning electrodes X1, X2, . . . , Xn are repeatedly scanned in the order named in a time-multiplexed manner. When a given scanning electrode (e.g., X3 in FIG. 41 is selected, a selection or nonselection display signal is simultaneously applied to all pixels P31, P32, and P3m on the given scanning electrode through the signal electrodes 33 constituted by electrodes Y1, Y2, . . . and Ym in accordance with a display signal. In other words, the on/off operation of the pixels at the intersections of the scanning electrodes and the signal electrodes is determined by a combination of voltage pulses applied to the scanning and signal electrodes. In this case, the number of scanning electrodes X corresponds to the number of time-multiplexing.
When the TN-LCD is driven by the amplitude-selective addressing scheme (as described in U.S. Pat. No. 3,976,362 to Kawakami), the ratio of the rms voltage exerted on the "on" (selected) segment to that exerted on the "off" (non-selected) segment decreases and the display contrast declines in proportion as the number of time-multiplexing cycles increases. In order for the number of time-multiplexing cycles to be increased without a sacrifice of display contrast, the slope of the luminance-voltage characteristic curve must be increased appreciably in steepness. The question as to how the steepness of the variation caused on luminance by voltage should be improved poses as an important technical task on the way toward successful commercialization of highly time-multiplexed LCD's.
A study is being energetically promoted in search of improvements in and concerning characteristic properties of liquid crystal materials themselves with a view to enhancing the steepness of this variation of luminance. While this study has borne fruits in bringing about improvements of a measure, it has not yet succeeded in perfecting a material satisfying ample steepness. As another means of improving the steepness of variation of luminance, there has been widely adopted the method which consists in creating a deviation between the liquid crystal molecular axes and the polarization (or absorption) axes of the polarizing plates in the regions adjoining the surfaces of the upper and lower electrode substrates so as to satisfy the relationship that the twist angle, .alpha., of the liquid crystal molecules is greater than the intersecting angle, .theta., of the polarization or absorption axes of the plarized plates.
Japanese Unexamined Patent Publication (Kokai) SHO 53(1978)-134,458, for example, discloses a configuration wherein the twist angle, .alpha., is selected in the range of 94.degree. to 106.degree. and the intersecting angle, .theta., of the polarizing axes of the polarized plates in the range of 74.degree. to 86.degree.. Japanese Unexamined Patent Publication (Kokai) SHO 55(1980)-95,929 discloses a configuration wherein for the purpose of enabling the intersecting angle between the polarization axes or absorption axes of the polarized plates to be smaller than the twist angle, .alpha., of the liquid crystal molecules, the intersecting angle between the polarization axes or absorption axes of the polarized plates is selected among 80.degree., 85.degree., and 90.degree. where the twist angle, .alpha., of the liquid crystal molecules is 85.degree. or the aforementioned intersecting angle between the polarization axes or absorption axes of the polarized plates is selected among 85.degree., 90.degree., and 95.degree. where the twist angle, .alpha., of the liquid crystal molecules is 90.degree.. Japanese Unexamined Patent Publication (Kokai) SHO 56(1981)-92,518 discloses a configuration wherein the twist angle, .alpha., of the liquid crystal molecules is selected in the range of 80.degree. to 100.degree. and the intersecting angle of the polarization axes of the polarized plates in the range of 74.degree. to 84.degree.. Japanese Unexamined Patent Publication (Kokai) SHO 59(1984)-40,623 discloses a configuration wherein the twist angle, .alpha., of the liquid crystal molecules is selected in the range of 92.degree. to 120.degree. and the intersecting angle, .theta., between the polarization axes of the polarized plates is selected so as to satisfy the formula, 180.degree..ltoreq..alpha.+O.ltoreq.210.degree.. None of the configurations mentioned above simultaneously satisfies the requirements concerning the steepness of the luminance-voltage characteristic slope, the phenomenon of coloration of display surface, and the viewing angle dependency of luminance.